This invention relates generally to electrophotographic printing and more particularly it relates to an image treatment method and apparatus for fusing toner images, with special emphasis on a Release Agent Management (RAM) system therefor.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the color toner material occurs causing the toner material to be bonded firmly to the support member. One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. To further enhance release, a release agent material such as silicone oil is applied to elastomer coating.
A RAM system comprising a metering roll and a donor roll has been utilized effectively in commercial imaging apparatuses. With such a system, it is customary to use a metering blade to meter the silicone oil or other suitable release agent material to a desired thickness onto a metering roll. In the fusing of monochrome (i.e. black on a conventional imaging substrate) the uniformity of the oil layer on the metering roll is not so critical compared to that required for color toner images, particularly, those associated with transparency substrate materials used for optically projecting the color images. The silicone oil is metered onto the roll using one or more metering blades.
The use of a metering blade that is contaminated with dirt or which has defects can result in the release agent oil being nonuniformly metered onto the metering roll. Such nonuniform application of oil can change the gloss characteristics of the color images and/or adversely affect the projection of color toner images carried by a transparency by creating streaks.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
One method of applying a release agent such as silicone oil utilizes a combination donor/metering roll arrangement wherein the metering roll contacts silicone oil in a sump and conveys a non metered amount to the metering blade. The metered layer of oil on the metering roll is transported to the donor roll and subsequently to a heated fuser roll. One such Release Agent Metering (RAM) system for applying silicone oils to a heated fuser roll is illustrated in U.S. Pat. No. 4,214,549. The system disclosed therein comprises a donor roll fabricated from a heat insulative and deformable material, for example, silicone rubber which transfers functional release material from a metering roll contacting a supply of release material contained in a sump to the heated fuser roll. A metering blade is supported in contact with the metering roll for metering the release material onto the metering roll to a specified amount per copy. This type of RAM system dispenses a fixed amount of release agent material to the fuser roll member.
In U.S. Pat. No. 5,200,786 granted to Fromm et al on Apr. 6, 1993, the donor roll of the '549 patent is replaced with a donor brush. As set forth in the '786 patent, the brush donor structure allows for the application of variable amounts of release agent material depending on the mode of operation. In other words, when color prints are being created a greater quantity of silicone oil is applied to the fuser roll compared to the amount applied when operating in the monochrome black mode.
A release agent management system is disclosed in Xerox disclosure Journal, Vol. 3, Number 6, November/December 1978. As disclosed therein, the RAM system comprises a metering roll to which silicone oil is applied or metered using a pair of blades. The metering roll is disposed such that it can be rotated through silicone oil contained in a sump. A first metering blade is supported for contact with the roll in a position below a second metering blade. The first metering blade is mounted slightly above the fluid level of the silicone oil contained in the sump. By tandem mounting the two blades less frequent maintenance is required because there is double the area for toner or dirt accumulation. By such orientation of these blades, the device is less dependent upon a tall curtain of oil, thus allowing a minimum static height which minimizes potential sloshing problems when the machine containing the device is moved about. The blade serves to pre-meter a fixed amount of oil which can subsequently be precision metered to the roll by the second blade. Thus, a first thickness of oil is metered to a lesser thickness by the second blade.